Support SVG sources (closes #68)

The fetcher accepts image/svg+xml again and internal/magic detects SVG.
A raster format renders the SVG at the least size that covers the
output, held to 8192 on a side, through libvips' thumbnail, which
renders at any scale, so an enlarged SVG stays sharp. An SVG's own size
is not limited; one with a side more than 8192 times the other is
ErrSVGTooThin, answered 415. orig of an SVG at the orig size is the SVG
unchanged, cached as a variant; at any other size it is
ErrSVGNotOriginalSize, answered 400. The cache reads up to 1024 first
bytes of a file to find its format. Render time and memory grow with
what the SVG draws: P1 in TODO.md. Tests that pinned SVG as refused now
expect it to be read.

Model: opus-5-5

internal/handlers/image_errors_internal_test.go

@@ -467,7 +467,7 @@ func TestImageRoutes_SameErrorAnswers(t *testing.T) {
 		{"no free upstream connection", httpfetcher.ErrTooManyConnections,
 			http.StatusServiceUnavailable, slog.LevelWarn},
 		{"Content-Type not a source format",
-			fmt.Errorf("%w: image/svg+xml", httpfetcher.ErrInvalidContentType),
+			fmt.Errorf("%w: image/tiff", httpfetcher.ErrInvalidContentType),
 			http.StatusUnsupportedMediaType, slog.LevelWarn},
 		{"any other failure", io.ErrUnexpectedEOF,
 			http.StatusInternalServerError, slog.LevelError},